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In Section 4.3, it is shown that(x,t)=Ae颈(办虫-蝇t)satisfies the free-particle Schr枚dinger equation for allxandt, provided only that the constants are related by(k)2/2m=. Show that when the function,(x,t)=Acos(kx0x)is plugged into the Schrodinger equation, the result cannot possibly hold for all values of x and t, no matter how the constants may be related.

Short Answer

Expert verified

We can't come up with a condition on or k that will make the proposed solution valid in this situation under any circumstances.

Step by step solution

01

Step 1:Schrodinger equation.

So, let's take the above function and plug it back into the Schrodinger equation to see if we can come up with a condition on the value of or k that will allow us to get this result.

22m2x2=it鈥︹赌︹赌︹赌︹赌︹赌(1)

02

Schrodinger equation for the given wavefunction

The wavefunction given is

=Acos(kx-蝇t)鈥︹赌︹赌︹赌︹赌︹赌.(2)

Substitute the value of wavefunction from equation (2) into equation (1), and we get,

2x2=Ak2cos(kx-蝇t)t=A蝇sin(kx-蝇t)

22m(-Ak2cos(kx-蝇t))=iA蝇sin(kx-蝇t)k22m=i蝇tan(kx-蝇t)

03

Conclusion.

Unlike the exponential case, this final expression has a function that depends on both position and time, but the left-hand side is constant. As a result, we can't discover a condition on kor that will make the two sides equal, and the sine function as a solution is no different.

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Most popular questions from this chapter

One of the cornerstones of quantum mechanics is that bound particles cannot be stationary-even at zero absolute temperature! A "bound" particle is one that is confined in some finite region of space. as is an atom in a solid. There is a nonzero lower limit on the kinetic energy of such a particle. Suppose minimum kinetic energy of width L. Obtain an approximate formula for its minimum kinetic energy.

Because we have found no way to formulate quantum mechanics based on a single real wave function, we have a choice to make. In Section 4.3,it is said that our choice of using complex numbers is a conventional one. Show that the free-particle Schrodinger equation (4.8) is equivalent to two real equations involving two real functions, as follows:

-221(x,t)m=2(x,t)tand

-222(x,t)m=1(x,t)t

where (x,t)is by definition 1(x,t)+i2(x,t). How is the complex approach chosen in Section4.3more convenient than the alternative posed here?

In the hydrogen atom, the electron鈥檚 orbit, not necessarily circular, extends to a distance of a about an angstrom (1脜=0.1鈥塶尘)from the proton. If it is to move about as a compact classical particle in the region where it is confined, the electron鈥檚 wavelength had better always be much smaller than an angstrom. Here we investigate how large might be the electron鈥檚 wavelength. If orbiting as a particle, its speed at 1脜could be no faster than that for circular orbit at that radius. Why? Find the corresponding wavelength and compare it to1脜 . Can the atom be treated classically?

The average kinetic energy of a particle at temperatureTis32kBT. (a) What is the wavelength of a room-temperature (22掳颁)electron? (b) Of a room-temperature proton? (c) In what circumstances should each behave as a wave?

The uncertainty in the position of a baseball of mass0.145kgism.What is the minimum uncertainty in its speed?

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